Chapter 2 Experimental Investigations
2.4 Preparation and characterization of nano-fluid
Nano-fluids are engineered colloidal suspensions of tiny nanometer-sized (anyone principal dimension < 100nm) solid additives called nano-particles in the base fluids, like water, mineral oil, vegetable oil, ethylene glycol, or their proportionate mixture [Taylor et al., 2013]. Different carbon-based, ceramic, metallic, semiconductor type, polymeric and lipid-based nanoparticles and a mixture of two nanoparticles can be used for fabricating nano-fluids. Nano-fluids are considered as 'smart fluid' due to their augmented thermo- physical properties and heat transfer characteristics, which can be controlled by controlling the nanoparticle concentration in the base fluid [B. Li et al., 2017]. Nanofluids were first proposed by Choi and Eastman [1995] and have become very popular at present. Thermal performance, such as thermal conductivity and the convection heat transfer coefficient of the nanofluid can be largely improved compared to simple base fluids [Yu et al., 2008].
Improved load-carrying capacity, anti-wear and friction reduction properties of the lubricating oils with nanoparticle additives have been revealed by various tribology researches [Jatti and Singh, 2015; Yu and Xie, 2012]. For the aforementioned properties, nano-fluids have become very popular in some cooling and/or lubricating application in a wide variety of applications including manufacturing, transportation, energy, and electronics, etc. The performance of nanofluids can be further improved by dispersing two
different types of nanoparticles in the base fluid, which is called hybrid nano-fluid. Due to synergistic effect of different nano particles, hybrid nano-fluid comprises better thermal characteristics compared to conventional nano-fluid. A trade-off between advantages and disadvantages of the single nano-fluids was happened when hybrid nano-fluid is prepared [Minea and Moldoveanu, 2018]. Although nanoparticles are very small in size, they have a high surface area to volume ratio. Due to their large surface area, their heat-dissipating capacity increases to a great extent.
There are several techniques for the fabrication of nano-fluids, such as a single-step method and a two-step method [Yu and Xie, 2012]. In the single-step method production of nanoparticles and their dispersion into a base-fluid to make nano-fluid is done in a single step by the direct evaporation process. Here, under vacuum condition source material is vaporized followed by a vapor that is condensed via contact between the vapor and a flowing liquid [Choi and Eastman, 2001]. By maintaining the continuous liquid flow nanoparticle agglomeration can be minimized and good dispersion can be assured.
However, this method is very expensive and the volume is also limited due to the limited space in the vacuum chamber. Recently, the two-step physical process is the most commonly used technique of nano-fluid preparation. In this process, nanoparticles are synthesized first in the form of dry powder. Nowadays, various nanoparticles are commercially available and can be used for making nano-fluid. The inert gas condensation technique has already been established to commercially produce huge quantities of nano- powders [Suryanarayana and Prabhu, 2006]. In the second processing step, the manufactured or purchased nano-particles are dispersed into an appropriate base fluid such as water, ethylene glycol and engine oil, etc. For mixing magnetic stirring can be used.
Due to enhance the stability of the nano-fluid addition of surfactant or ultra-sonication method can be used. This method is more economical than the one-step method due to the comparatively low price of available nanoparticles. However, the nanofluid prepared by this method usually has a stability problem. All the nanofluids slowly settle down after a period based on the nano-particle type, base fluid, surfactant used, ultrasonication/
magnetic stirring time. The useful time of the nanofluid is before sedimentation.
In this research, Hybrid Al2O3-MWCNT based nanofluid has been used as MQL fluid. MWCNT has higher thermal conductivity (3000W/m K) which enhance the thermal conductivity significantly and provide best heat transfer performance. On the other hand,
nano-particles and provide better surface quality. So, Al2O3-MWCNT based nanofluid has higher heat transfer and lubrication property. As mentioned earlier, the two-step physical process is the easiest one and also the least expensive. So, it has been used to synthesize hybrid Al2O3-MWCNT based nanofluid. The stability of a single MWCNT based nano- fluid can be increased by the addition of an oxide nano-particle with it [Yang et al. 2020].
So, the stability of Al2O3-MWCNT hybrid nano-fluid should be more than a single MWCNT based nano-fluid. The MWCNT was purchased from Tanfeng Tech. Inc. China, which has a mean diameter of 10-20 nm and a length of 5-15 μm. MWCNT has a density of 2.1 gm/cm3. The Al2O3 nanoparticle powder has been purchased from Sigma-Aldrich Chemicals Pvt. Ltd, INDIA, which has a mean diameter of <50 nm and has a density of 3.9 gm/cm3. Hybrid nano-fluid with a concentration of 0.5%, 1% and 1.5% were prepared by dispersing 80% Al2O3 and 20% MWCNT into the base fluid. Generally, more Al2O3
volume proportions provide better absorbance of hybrid nano-particles into a base fluid [Asadi et al., 2018]. The weight of nanoparticles required was calculated by using Eq.
2.13:
...(2.13)
where ρ is the density in gm/cm3and m is the mass in gm. The masses of Al2O3 and MWCNT nanoparticles used for preparing a volume of 100 ml nanofluid were determined and presented in Table 2.10.
Table 2.10 Masses of nano-particles used for preparing 100 ml of nanofluids
Nano-particle concentration, % Mass (gm)
MWCNT (20%) Al2O3 (80%)
0.5 0.21 1.57
1 0.42 3.15
1.5 0.64 4.75
For the preparation of specific concentration nanofluid, the required amount of commercially available Al2O3 and MWCNT nanoparticles have been dispersed into the VG 68 cutting oil. Then 2 hours of ‗Magnetic stirring' with the rotational speed of 700 rpm were used to mixing the nanoparticle with the oil followed by 1hr of 'Ultrasonic agitation' with the frequency of 20 kHz for preventing the agglomeration of the fluid. Both of these above-mentioned methods were used by several researchers due to preparing stable nanofluids [Krishna and Rao, 2016]. The duration of these processes is an important
factor, on which the thermal properties of nanofluid depends [Leong et al., 2016]. The Nano-fluid preparation procedure is shown in Fig.2.17.
Base fluid-VG 68
Al2O3
nanoparticle
MWCNT
Magnetic
stirring Ultrasonic agitation
Al2O3-MWCNT hybrid nano-fluid
Fig.2.17 Nano-fluid preparation procedure
No surfactant was used in this experiment because for oil-type base fluid it is not essential for stable dispersion. Moreover, the surfactant can create fumes when applying the nano-fluid at the high-temperature zone and also affect the thermal properties of the nano-fluid. MWCNT nanofluid sample was kept for observation and no visible settlement of particle was not observed even after 3 hours. The hybrid nano-fluid prepared are assumed to be isentropic, Newtonian in behavior and their thermo-physical properties are uniform and constant with time all through the fluid sample.
Thermal conductivity is a very significant property of nanofluid, used as a coolant.
Different parameters affect the thermal conductivity of nanofluids, such as the type of particle and the base fluid [Wang et al., 1999], the morphology and concentration of particle [Mirmohammadi et al., 2019], temperature [Das et al., 2003], additives used or not [Eastman et al., 2001] clustering of the particle [Zhu et al., 2006], the acidity of nanofluid [Xie et al., 2002] and so on. Morphology of the particle means shape, size, texture, etc.
Due to the enhancement of thermal conductivity of nano-fluid compared to a base fluid, nanofluid is attracted by the sectors where the cooling action is very necessary. It is expected that any solid particle usually enhances the thermal conductivity of the fluid.
Because solid particle has greater thermal conductivity compared to liquid.
There are several methods used for measuring the thermal conductivity of nanofluids, such as transient hot-wire techniques, the steady-state parallel-plate method, cylindrical cell method, temperature oscillation technique, etc [Paul et al., 2010]. The
measuring the thermal conductivity of fluids. In this technique, two coaxial cylinders are used, where a heater is placed inside the inner cylinder and the outer cylinder is cooled by water. The fluid is kept in between the cylinders. Using Fourier‘s law with the temperature of the coaxial cylinders, the thermal conductivity of the liquid samples in the gap could be calculated. Plug and jacket type apparatus is based on the concept of cylindrical cell method, which was previously used for measuring the thermal conductivity of the nanofluids [Vajjha and Das, 2009]. In this research, plug and jacket type bench-top apparatus (H471) supplied by P.A. Hilton Ltd has been used, which is completed with a console for the control and display of temperature and heat input. Here, the inner cylinder is called a plug and the outer cylinder is called a jacket. In between the heated plug and a water cooling jacket, there is a small radial clearance (0.3mm). This small clearance is filled by the fluid, whose thermal conductivity is needed to be examined. The clearance is very small, so no natural convection has happened to the working fluid. The fluid is there as a form of a lamina of face area πdmL (0.133m2) and thickness of ∆r (0.3mm) and transfers heat from the plug to the jacket.
For minimizing thermal inertia and temperature variation the plug is manufactured from aluminum anodized (mean diameter 39 mm, effective length 110 mm). A cylindrical heating element of resistance 55 Ω (may vary for different apparatus) is used to heat the plug. For measuring the temperature, a thermocouple is inserted into the plug close to the external surface. The plug has an inlet and an outlet for the test fluid and the fluid is injected by a syringe. By using two 'O' rings the plug is held centrally in the water jacket.
For the cleaning purpose 'O' rings can be easily removed. The cylindrical jacket is manufactured from nickel-plated brass which has a water inlet and an outlet. The jacket is cooled by a continuous supply of water at a rate of 3 liters/min. A K-type thermocouple is attached to the inner sleeve of the jacket. The thermocouples have measured the temperature of the hot and cold faces of the test fluid lamina. A small console (aluminum and plastic coated) is connected to the plug/jacket assembly and controls the voltage supplied to the heating element. To determine the power input an analog voltmeter (0- 60V) is used and a digital temperature indicator with a selector switch displays the temperature of the plug and jacket with 0.1o C resolution. Calibration of the equipment by a fluid with known thermal conductivity is necessary before evaluating the sample.
For thermal conductivity measurement of hybrid Al2O3-MWCNT nano-fluids, the plug and jacket assembly unit was dismantled, cleaned and reassembled. The nanofluid
was then injected into the radial space of the apparatus. Sufficient liquid must be passed through the clearance space such that there is no air pocket. Water was passed through the jacket and the heater adjusted to give a temperature difference and heat transfer rate. when stable the voltage and the plug and jacket temperatures were observed. The incidental heat transfer at the given temperature difference is deducted from the total heat input and considering that the rest was passing through the fluid lamina.
Here,
t1 = Plug temperature t2 = Jacket temperature
∆t =Temperature difference V= Voltage applied to the heater R= Heater resistance
∆r = Radial clearance
A (πdmL) = Heat transfer area
Electrical heat input, Qe= ………...……...…….…..…...(2.14) Incidental heat transfer at ∆t (from the graph) = Qi
Heat conduction through oil, Qc=Qe- Qi …...……...………...…..…...(2.15) Thermal conductivity of oil, K = ………...……...…….…..…...(2.16) The thermal conductivities of different nanofluid samples calculated are shown in Fig.2.18.
25 30 35 40 45 50 55 60 65
0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28
0.30 Base fluid
0.5 % Al2O3/MWCNT 1.0 % Al2O3/MWCNT 1.5 % Al2O3/MWCNT
Temperature, OC
Thermal conductivity, k (W/mK)
Fig.2.18 Thermal conductivity of the base fluid and nano-fluids
Dynamic viscosity is a physical property of a fluid that indicates its resistance to flow or shear and a measure of fluid's adhesive/cohesive or frictional properties. It is a very important property of fluid when analyzing liquid behavior and fluid motion near solid boundaries. Nanofluid viscosity was not investigated thoroughly as the other thermal properties, but it is one of the most critical parameters of nanofluids. For the cooling application of nanofluid, thermal conductivity increment is necessary. On the other hand, less increment or constant viscosity is more acceptable, which indicates a higher ratio of thermal conductivity and viscosity. Some crucial factors like temperature, nanoparticle volume fraction, particle size, morphology, dispersion method, etc. influence the viscosity of nanofluids [Mahbubul et al., 2012; Mishra et al., 2014].
There are several techniques for evaluating the viscosity of nano-fluids, such as rotational viscometer [Pastoriza-Gallego et al., 2011], vibration viscometer [Lee et al., 2011], falling ball viscometer [Feng and Johnson, 2012], efflux cup viscometers [Kumar et al., 2018; Seyedzavvar et al., 2019], etc. In this research efflux cup type say-bolt universal viscometer (Koehler- SV3000), has been utilized for the measurement of viscosity of all the samples. In this viscometer, the temperature of the liquid holding vessel and the orifice is controlled by submerging them in a thermostatically controlled oil bath. Test liquid is placed in the vessel and the temperature is set. The temperature can be set between 21 to 99°C. When the temperature reaches the specific value, the orifice is opened. The viscosity is the efflux time in seconds, required by the fluid to pass from a vessel through an orifice under precisely controlled conditions to fill a 60 cc container. Time is counted by using a stopwatch. The universal orifice has dimensions of 0.176 cm in diameter and 1.225 cm in length. The efflux time required by using a universal orifice is called Say bolt universal seconds (SUS), and it is a measure of viscosity.
Say bolt universal seconds (t) can be converted to kinematic viscosity (v) by using Eq. 2.17 and Eq. 2.18:
When,
...….(2.17) ...(2.18)
For all the samples kinematic viscosity decreased with the increase of temperature.
Also with the increase of nanoparticle concentration from 0.5 to 1.5 vol % viscosity has been increased. Kinematic viscosity of the base fluid and nano-fluids has been presented in Fig.2.19.
25 30 35 40 45 50 55 60 65
0 20 40 60 80 100 120 140 160 180 200
Temperature, OC Kinematic viscosity,centistikes) Base fluid
0.5 % Al2O3/MWCNT 1.0 % Al2O3/MWCNT 1.5 % Al2O3/MWCNT
Fig.2.19 Kinematic viscosity of the base fluid and nano-fluids